It is known that sponge iron can be produced by reducing fine-grained iron ores in a fluidized bed reactor with gaseous reduction agents, such as, for example carbon monoxide and hydrogen. The metallic iron contained in the so-produced fine-grained sponge iron is easily reoxidized. Therefore, the fine-grained sponge iron must be protected against reoxidation before it is melted (which melting is usually a discontinuous process) by changing it into a solid form by means of so-called hot briquetting or by storing and transporting it in inert gas. Hot briquetting, as well as storage and transport in inert gas, produce technical difficulties and substantial costs. Further, reoxidation can never be completely prevented by either of these protective measures because such reoxidation also takes place, to a certain extent, at the surface of the sponge iron briquettes and the inert gases employed always have a slight oxygen content.
In order to be able to effect the melting of the sponge iron at a justifiable cost, the sponge iron must contain as large an amount as possible of metallic iron. The high degree of metallization of the sponge iron desired in the reduction of the ore produces high costs, however, and requires particularly effective protective measures to prevent reoxidation.
It is also known to melt sponge iron by adding electrical energy or combustion heat in a suitable apparatus. Natural gas, crude oil and coal can be used to produce the combustion heat. Suitable melting devices are hearth furnaces, (electric arc furnaces, Siemens-Martin furnaces), shaft furnaces (blast furnaces, cupola furnaces, electric low-shaft furnaces) and crucibles (oxygen refining converters) in which in addition to the melting process, alloying, final reduction and/or refining processes can also take place. Thus it is possible, for example, to melt sponge iron in a blast furnace and at the same time subject it to a final reduction, the end product being pig iron which is rich in carbon. When pig iron is refined in a converter, the carbon contained in the pig iron to an amount up to 4% is burned by the blown-in air or oxygen and the heat produced by this process can be utilized to melt the sponge iron. The capacity of the converter for sponge iron to be melted is undesirably limited, however, by the carbon content of the pig iron in the converter. Further, the nozzles with which the oxygen is introduced into the liquid pig iron are subject to heavy mechanical, thermal and chemical stresses which lead to malfunctions in the refining and melting processes. Many attempts have therefore been made to introduce larger quantities of heat into the converter by suitable measures and to reduce the stresses inherent in the process to which the oxygen injection devices are subjected. In one such prior art method, the metal bath in the converter has been heated by an oil heating system operated with oxygen, but this method has not found acceptance because the capacity of the converter for sponge iron to be melted could not be substantially increased in view of economical considerations due to insufficient heat transfer from the combustion gases to the metal bath.
It is also the custom to introduce the sponge iron to be melted into the converter in charges and to remove the molten steel present after the refining process in a discontinuous manner so that longer starting and dead times are encountered for the converter.